The invention is based on a valve control apparatus with a magnet valve for internal combustion engines as defined hereinafter.
In an already-proposed valve control device for controlling the closing and opening time of an engine valve actuated by the valve control cam of a camshaft via an axially displaceable valve shaft (German Patent 38 15 668.7 now U.S. Pat. No. 4,889,084), the fluid reservoir is integrated with the magnet valve; the valve member serves as the reservoir piston, which divides a reservoir chamber from a magnet chamber, and the connection between the valve inlet and the reservoir chamber is controlled via one face edge of the reservoir piston in cooperation with the valve seat. The magnet acts counter to the direction of reservoir deflection, because in the specialized exemplary embodiment the magnet valve is intended to be open when without current, or in other words to be blocked only under voltage. This is intended to assure that the engine cannot race if the plug falls off the magnet valve. However, the consequence is that if the plug does fall off, the engine stays stopped. To attain the above proposed embodiment of the invention, in which on the one hand the fluid reservoir is integrated with the magnet valve and on the other hand the magnet valve is intended to be open when without current, entails not inconsiderable expenditure, in particular for construction, because the mechanical adjusting forces--that is, the reservoir force on the one hand and the opening force on the other--of the magnet valve counteract one another, so that at least two springs must engage the reservoir piston serving as the valve member. Not only must the necessary space for this be available, but the two springs must also be tuned very accurately to one another, which is especially difficult to achieve since this tuning must also take into account the forces exerted by the hydraulic pressures. Thus the static pressure, which engages the reservoir piston on the part of the reservoir spring, must necessarily be lower than the hydraulic pressure that is exerted by the engine valve and engages the reservoir piston. Only in this way is the desired reservoir effect possible. On the other hand, however, the static pressure must be greater than the oil supply pressure with which leakage and exclusion losses in the hydraulic oil from the valve control device are compensated for. Only in this way is it assured that upon reduction of the engine valve pressure, the reservoir piston will return to its outset position, from which it can be moved in the valve closing direction upon excitation of the magnet, or from which it can conversely deflect, upon reduced excitation and a buildup of tappet pressure.
To meet the requirements of both safety and comfort in a motor vehicle, the above safety shutoff means is inadequate in the event of loss of a plug from the magnet valve. If the plug drops off in this way, the situation in which the magnet valve sticks in the excited state cannot in fact be prevented, so engine racing is possible. Accordingly, it is indispensable to have additional devices that switch off the supply or ignition of fuel.